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BUREAU OF MINES u , « 

INFORMATION CIRCULAR/1989 




Ultra-High-Purity Silicon for Infrared 
Detectors: A Materials Perspective 



By Clark R. Neuharth 



UNITED STATES DEPARTMENT OF THE INTERIOR 



Mission: Asthe Nation's principal conservation 
agency, the Department of the Interior has respon- 
sibility for most of our nationally-owned public 
lands and natural and cultural resources. This 
includes fostering wise use of our land and water 
resources, protecting our fish and wildlife, pre- 
serving the environmental and cultural values of 
our national parks and historical places, and pro- 
viding for the enjoyment of life through outdoor 
recreation. The Department assesses our energy 
and mineral resources and works to assure that 
their development is in the best interests of all 
our people. The Department also promotes the 
goals of the Take Pride in America campaign by 
encouraging stewardship and citizen responsibil- 
ity for the public lands and promoting citizen par- 
ticipation in their care. The Department also has 
a major responsibility for American Indian reser- 
vation communities and for people who live in 
Island Territories under U.S. Administration. 



Information Circular 9237 



Ultra-High-Purity Silicon for Infrared 
Detectors: A Materials Perspective 

By Clark R. Neuharth 



UNITED STATES DEPARTMENT OF THE INTERIOR 
Manuel Lujan, Jr., Secretary 

BUREAU OF MINES 
T S Ary, Director 






^ 



Library of Congress Cataloging in Publication Data: 



Neuharth, Clark R. 

Ultra-high-purity silicon for infrared detectors : a materials perspective / by 
Clark R. Neuharth. 

p. cm. - (Bureau of Mines information circular; 9237) 

Bibliography: p. 12 

Supt. of Docs, no.: I 28.27:9237. 

1. Silicon. 2. Infrared detectors-Materials. I. Title. II. Series: Information 
circular (United States. Bureau of Mines); 9237 

TN295.U4 [TN948.S6] 622 s-dc20 [355.2'4] 89-600256 

CIP 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Semiconductors 2 

Properties and grades 2 

Semiconductivity 3 

Semiconductor devices 4 

Resources 5 

Recovery technology 5 

Silicon metal 6 

Silanes 6 

Polycrystalline silicon 7 

Single-crystal silicon 7 

Characterization 7 

Supply and demand 9 

Research and development 11 

Government programs 11 

Summary 12 

References 12 

Appendix-Major domestic and foreign firms involved in semiconductor silicon production 13 

ILLUSTRATIONS 

1. Face-centered cubic silicon lattice 3 

2. Semiconductivity of silicon 4 

3. Silicon junction devices 5 

4. Photodetector device 5 

5. Cross section of silicon metal furnace 6 

6. TCS production flowsheet 6 

7. Polysilicon reactor 8 

8. Single-crystal silicon production methods 8 

TABLES 

1. Semiconductor demand, by major market 3 

2. World silicon metal production 9 

3. U.S. silicon metal statistics 10 

4. Silicon supply-demand relationships and prices 10 

5. World semiconductor silicon suppliers 11 



UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 


in 


inch 


ppt part per trillion 


kg 


kilogram 


st short ton 


kg/yr 


kilogram per year 


yr year 


ppm 


part per million 





ULTRA-HIGH-PURITY SILICON FOR INFRARED 
DETECTORS: A MATERIALS PERSPECTIVE 



By Clark R. Neuharth 1 



ABSTRACT 

To assess the supply status of ultra-high-purity silicon for national defense needs, the U.S. Bureau 
of Mines conducted a general study of the availability of materials and processing technology for silicon 
used in the manufacture of infrared (IR) detectors from raw materials to the highly advanced device 
state. The United States possesses all of the raw materials and processing technology for IR detector- 
grade silicon production, but does not produce at all of the processing stages on a regular basis. 
Consequently, the United States continues to rely on foreign sources for some of these critical materials. 



1 Physical scientist, Division of Mineral Commodities, U.S. Bureau of Mines, Washington, DC. 



INTRODUCTION 



Silicon is one of the most abundant elements in the 
earth's crust. On a tonnage basis, it is used primarily as 
ferrosilicon (an alloy containing iron and silicon) for deox- 
idation and as an alloying agent in the production of iron 
and steel. Metallurgical-grade silicon metal is also used on 
a tonnage basis in aluminum alloys and by the chemical 
industry as a feed material in the manufacture of silicones 
and silanes. Subsequently, silanes are used to manufacture 
high-purity silicon, which is the key material in today's 
eletronics industry. The actual amount of silicon that is 
ultimately processed to the point of usefulness in the man- 
ufacture of semiconductor devices is less than a few per- 
cent of total U.S. demand for silicon. The path that silicon 
must follow from a raw material state to the manufacture 
of certain semiconductor devices, such as infrared (IR) 
detectors, is quite complex and involves a number of pro- 
cessing stages. Currently, the United States is one of the 
world's leaders in semiconductor device manufacture, but 



does not produce on a regular basis all of the IR detector- 
grade materials needed by the Department of Defense for 
the manufacture of devices that guide highly advanced 
weapons and serve numerous functions as part of satellite 
surveillance systems. Capabilities to produce these ma- 
terials do exist in the United States, but owing to limited 
commercial applications, U.S. companies have been re- 
luctant to enter respective phases of the market. Conse- 
quently, the United States has relied almost entirely on 
foreign sources for some of these ultra-high-purity 
materials. 

By presenting a digest of the available sources of silicon 
materials and processing technology involved in the manu- 
facture of IR detector-grade silicon, a perspective can be 
drawn showing the overall importance of these materials 
and whether or not there is a need for concern from a 
national defense standpoint. 



SEMICONDUCTORS 



Although semiconductors serve thousands of functions, 
the demand for these devices can be generally grouped 
into three major categories: discrete semiconductors, 
integrated circuits (IC's), and optoelectronic devices 
(table 1). Discrete semiconductors include devices such as 
diodes, high-power transistors, and thyristors. IC's, the 
largest category based on total dollar value, include mem- 
ories, processors, custom and semicustom IC's, linear IC's, 
and logic devices. Imaging arrays, optically coupled iso- 
lators, and photodetectors, make up the bulk of optoelec- 
tronic devices. 

Optoelectronic devices account for less than one-tenth 
of one percent of U.S. semiconductor demand, but some 



of these devices are extremely important in "high-tech" 
military applications. IR detectors, a type of photodetector 
capable of converting IR radiation into an electrical signal 
(I), 2 are an essential part of a number of the military's 
weapons systems (e.g., "smart" missile guidance and sur- 
veillance satellites). The silicon starting material needed 
to produce IR detectors must meet very stringent purity 
requirements. Since most of these devices have almost no 
commercial applications, semiconductor silicon producers 
have been reluctant to enter this phase of the market (2). 



PROPERTIES AND GRADES 



Silicon never occurs free in nature, but is combined 
with oxygen and other elements to form oxides and sili- 
cates. Silicon dioxide (Si0 2 ), also referred to as silica, 
quartz, or sometimes quartzite, is one of the most common 
of these minerals. 

In its metallic form, silicon is the result of chemically 
reduced Si0 2 . The silica used to produce silicon metal is 
typically >99.0% pure (Si0 2 content by weight), with alu- 
minum, calcium, iron, and phosphorus constituting the 
bulk of impurity elements. This grade of starting material 
generally yields a silicon product that is 98% to 99% pure 
(Si content by weight). 



The crystal structure of silicon is diamond cubic, where 
each atom in the lattice shares its four outermost electrons 
(i.e., valence electrons) with the four nearest neighboring 
atoms through covalent bonding (fig. 1). This crystal 
structure, along with a number of other properties, gives 
silicon its edge as a mainstay among semiconductor 
materials. 



2 Italic numbers in parentheses refer to items in the list of references 
preceding the appendix. 



Table 1 .-Semiconductor demand by major market 

(Millions of current dollars, January 1989) 



Country 



Discrete 



Integrated 
circuits 



Opto- 
electronic 



Total 



1989 1 



$3,187 
2,142 
509 
302 
250 
187 



6,577 



Japan 

United States 

Germany, Federal Republic of 

United Kingdom 

France 

Italy 

Total 

Japan 

United States 

Germany, Federal Republic of 

United Kingdom 

France 

Italy 

Total 

Japan 

United States 

Germany, Federal Republic of 

United Kingdom 

France 

Italy 

Total 

Projected. 

Source: Electronics. 



Generally, the impurities contained in silicon for regular 
semiconductor device manufacture must be held in the 
low-ppm range, sometimes referred to as six 9's or 
99.9999% pure. For IR-detector -grade material, these 



$18,003 

13,853 

1,878 

1,385 

956 

704 



$2,585 

377 

134 

120 

81 

36 



$23,775 

16,372 

2,521 

1,807 

1,287 

927 



36,779 



3,333 



46,689 



1988 



$2,998 
2,113 
503 
302 
242 
175 



$16,214 

12,599 

1,783 

1,385 

870 

651 



$2,312 

361 

131 

120 

75 

33 



$21 ,524 

15,073 

2,417 

1,807 

1,187 

859 



6,333 



33,502 



3,032 



42,867 



1987 



$2,815 
1,822 
480 
277 
223 
154 



$12,837 

9,252 

1,522 

1,210 

676 

434 



$1,996 

332 

124 

108 

66 

31 



$17,648 

1 1 ,406 

2,126 

1,595 

965 

619 



5,771 



25,931 



2,657 



34,359 




Figure 1 .-Face-centered cubic silicon crystal lattice. 



impurity levels must be lowered to a range of 10 to 
100 ppt, or eleven 9's. The primary impurities of concern 
are the atoms of those elements that act as electron 
donors and acceptors (i.e., dopants) in a silicon lattice, 
such as boron and phosphorus (see Semiconductivity). As 
few as 10 or 20 ppt impurity can significantly alter the 
operation of a device such as an IR detector. Currently, 
there are only a few characterization methods that can 
detect impurities at these low levels, which is one of the 
primary concerns in the production process. 

SEMICONDUCTIVITY 

The structure of silicon and how it relates to semicon- 
ductivity can be represented by a two-dimensional diagram 
of the crystal lattice. Figure 24 shows a theoretically 
perfect, sometimes referred to as intrinsically pure, silicon 
lattice. At room temperatures, valence electrons can break 
free from the covalent bonds by acquiring energy from the 
internal heat energy or vibrations of the crystal lattice 
(fig. 2fl). These electrons hence become negative (n-type) 
charge carriers, whereas the voids or holes they leave 
behind become positive (p-type) charge carriers. The 
energy required to bring about this condition is called the 



• • • 

(7). .(T). .0 

» > » 

0. 0. 

• • • 

» > » 

0.0.0 




B 



• • • 

v Si y v Si y v si y * 

» » » 

v Si y v As y* *v Si y 

• • • • 

« • • 

• Tsi J. '(sij. •( Si V 



• • • 

• 0..0..0 

• 9 • 

• (si \ •( Al V 'fsij 

• o • 

» » » 



Figure 2.-Semiconductivity of silicon. A, No conductivity; 8, intrinsic; C, n- 
doped; D, p-doped. 



"gap" energy and varies with different materials. A mate- 
rial is considered an insulator when the gap energy is too 
great to be achieved. The conductivity exhibited in this 
basic example is totally inherent (i.e., not brought about by 
impurities) and is therefore known as intrinsic semicon- 
ductivity. Extrinsic semiconductors, sometimes referred to 
as doped semiconductors, contain intentionally introduced 
impurities called dopants, which alter the semiconducting 
characteristics of the material. Figure 2C shows how the 
introduction of a pentavalent atom (i.e., containing five 
valence electrons, such as phosphorus or arsenic) into the 
silicon lattice creates an n-type semiconductive condition 
similar to that shown in fig. 25. On the other hand, the 
introduction of a trivalent atom, such as boron or alumi- 
num, will create a p-type condition (fig. 2D) similar to that 
of the hole left behind in the example in figure IB (3). 

Whereas silicon exhibits semiconductive properties, Si0 2 
acts as an insulator and forms naturally on a silicon sur- 
face. This natural oxide-forming property allows con- 
trolled formation of Si0 2 layers on silicon substrates. 
These insulating layers serve a number of important func- 
tions in the processing (e.g., planar technology) and 



operation of semiconductor devices. Many other semicon- 
ductor materials do not possess this natural oxide-forming 
ability, making device manufacture and operation more 
difficult and sometimes impossible (4). 

SEMICONDUCTOR DEVICES 

Junction devices, such as diodes, are the combination 
of n-type and p-type semiconducting materials. In a simple 
pn junction (fig. 3/1), the n-type portion of the semicon- 
ductive material contains excess electrons, whereas the 
p-type portion is electron deficient. Between these two 
distinct regions lies a portion of material that possesses a 
negligible amount of charge carriers called the depletion 
layer. The depletion layer is void of charge carriers be- 
cause the excess electrons from the n-type material have 
crossed the junction to fill holes in the valence bands of 
the nearest p-type region. This flow of electrons ceases 
when the potential difference between the two portions 
reaches a certain magnitude (i.e., the barrier potential). 
The potential difference can be increased through the 
application of an external source of electromotive force 



Electrons Depletion layer Ho|es 




Light energy 




Figure 4.-Photodetector device. 



n-type 



p-type 



n-type 



p-type 





Negligible current 



Electron flow 



Figure 3.-Silicon junction devices. A, Diode and B, rectifier 
(adopted from Jackson). 



(emf). Current flow in the device is dependent on the 
direction in which the external source is applied (fig. 3fi), 
making the device function as a current rectifier (5). 

Similarly, pn junctions are capable of generating emf 
from electromagnetic waves, such as sunlight or IR radi- 
ation (fig. 4). When sufficient energy is passed through a 
transparent film of p-type silicon, excess electrons in the 
adjacent n-type region acquire the energy and migrate 
through the depletion layer into the p-type region, causing 
a current to flow through the external load. This principle 
forms the basis of operation for photodetectors. Probably 
the most common example of photodetectors are solar 
cells, which are used to convert sunlight into electrical 
energy. 



RESOURCES 



The Earth's crust is made up almost entirely of silica 
and silicates. These minerals constitute the bulk of most 
common rocks, sands, soils, and clays. Based on present 
requirements, domestic deposits of quartzite, sandstone, 
and pegmatitic quartz could sustain the U.S. ferrosilicon 
and silicon metal industries indefinitely. However, eco- 
nomic factors such as accessibility to low-cost energy, 
transportation costs, and a ready market for a product 



determine resource development. Since metallurgical- 
grade silicon is the primary raw material for the produc- 
tion of silanes subsequently used to manufacture semicon- 
ductor silicon, a shortage of raw materials is highly 
unlikely. However, pitfalls in the production of 
semiconductor silicon could occur if the capacity to 
complete any phase of the silicon production chain was 
interrupted. 



RECOVERY TECHNOLOGY 



There are a number of steps involved in the processing 
of semiconductor silicon. Some companies involved in the 
semiconductor industry are fully integrated, while others 
specialize in just one or two phases of the processing 
chain. The basic raw material for semiconductor silicon is 
quartzite (Si0 2 ), which is abundant and for the most part 
mined and processed domestically. The initial processing 
step involves the reduction of quartzite to metallurgical- 
grade silicon (sometimes referred to as chemical-grade 



silicon in the chemical and electronics industries, or simply 
as silicon metal). Quartzite used to produce silicon metal 
is generally >99.0% pure. Following quartzite reduction, 
the silicon metal is converted to a silicon-based chemical 
that can be reduced to a purified silicon material in poly- 
crystalline form. This polycrystalline silicon is then further 
processed into a form that possesses the desired semicon- 
ductive properties for device manufacture (i.e., single 
crystal). 



SILICON METAL 



SILANES 



The principal raw material for silicon metal production 
is beneficiated Si0 2 in the form of quartzite or certain 
sandstones. The silica is reduced with carbon in a 
submerged-arc electric furnace. The overall reaction is as 
follows: 



SiQ 2 + 2C 



Si + 2CO 



proceeding to the right above 1,164° C. However, in prac- 
tice, temperatures vary in different furnace locations, and 
a number of side and/or intermediate reactions also occur. 
The products of these reactions subsequently migrate to 
regions of the furnace where they react further or exit the 
process (fig. 5). The silica starting material (i.e., quartzite) 
is typically >99.0% pure, with aluminum, calcium, iron, and 
phosphorus constituting the bulk of impurity elements. 
Any iron contained in the silica will be reduced and report 
to the metal. The amounts of aluminum, calcium, and 
phosphorus present in the metal after reduction range 
from 40% to 70% of their original content (6). 

In the United States, silicon metal production of this 
type ranges from 120,000 to 150,000 st annually. However, 
demand is normally 30,000 to 40,000 st higher than produc- 
tion, making the United States a net importer of silicon 
metal. 



Charge materials 
SIO, + 2C 



r~ 




's 



♦ ♦ 
Combustion 

CO— CO, 

SI0 — SI0, 



H^4MNU 



si -£"« 



/ 



-\-l 1 



L*-2C + SIO 




Compounds containing hydrogen-silicon bonds are typ- 
ically classified as silanes or sometimes as silicon hydrides. 
Silane (SiH 4 ) is the simplest form of these compounds. 
Other forms are named with the substituents prefixed, 
such as disilane, H 3 SiSiH 3 ; dichlorosilane, H 2 SiCl 2 ; and 
trichlorosilane (TCS), HSiCl 3 . TCS and silane are the pre- 
ferred compounds for the production of polycrystalline 
silicon. 

TCS is produced by the reaction of powdered metal- 
lurgical-grade silicon with anhydrous hydrogen chloride 
(HC1) in a fluidized bed. The general equation for the 
reaction is 



Si + 3HC1 



HSiCl 3 + H 2 . 



However, a number of products other than TCS and H 2 
are formed, including silicon tetrachloride (SiCl 4 ), other 
chlorosilanes, unreacted HC1, and various metal chlorides 
(A1C1 3 , BC1 3 , PC1 5 , etc.). The liquid TCS is separated from 
the other products and purified by fractional distillation 
(fig. 6) (7). 

Silicon powder 
HCI 



Fluidized-bed 
reactor 



HCI H, 



Recovery 



\ 

HCI 

H 2 



TCS 
SiCI. 

HCI 
H 2 



Separation 



TCS 
SiCI 4 



Distillation 



Figure 5.-Cross section of silicon metal furnace. 



" 
TCS SiCI 4 

Figure 6.-TCS production flowsheet. 



Silane is produced by a number of methods involving 
the reaction of metal silicides (e.g., those of aluminum, 
lithium, and magnesium) with acids or ammonium salts. 
Another silane process known to be used for commercial 
production in the United States involves the conversion of 
silicon metal to TCS, followed by the catalytic redistri- 
bution and distillation of chlorosilanes (8). 

POLYCRYSTALLINE SILICON 

Polycrystalline silicon rods, often referred to as poly- 
silicon or simply poly, are produced commercially by two 
methods: (1) chemical vapor deposition from TCS in the 
presence of hydrogen (i.e., the reverse of the fluidized-bed 
reaction in TCS production), and (2) thermal decompo- 
sition of silane. A simplified equation of the reaction of 
TCS and hydrogen would be 



HSiCl 3 + H 2 



Si + 3HC1. 



However, the actual process also yields SiC14 as a by- 
product as well as unreacted TCS and H 2 . The ability to 
recover and reuse the vent products plays an important 
economic role in the production cycle (9). A typical de- 
composition reactor containing a polysilicon filament (i.e., 
starting rod) is shown in figure 7; thermal decomposition 
of silane is carried out in a similar reactor. In this process, 
reaction temperatures are much lower than those needed 
for the TCS-hydrogen reaction, resulting in increased 
polysilicon yields, lower impurity levels, and reduced pro- 
duction costs through decreased power consumption (10). 
A third method, also involving silane decomposition, is 
used to produce polysilicon shot in a fluidized-bed reactor. 
However, this material cannot be converted to single- 
crystalline form using float-zoning (FZ) techniques. 

SINGLE-CRYSTAL SILICON 

Since polysilicon contains structural defects that would 
affect electrical properties and interfere with semicon- 
ductor device manufacturing processes, it must be con- 
verted to a single-crystal form prior to device manufacture. 
Single-crystal boules (rods) are grown by either the 
Czochralski (CZ) or the FZ method. These boules are 
later sliced into wafers (flat discs). In the CZ method 
(fig. &4), a seed crystal is touched to the surface of a 
molten silicon charge, and a solidified single crystal grows 
as the seed is slowly pulled away from the melt. The 



pulling rate controls the diameter of the boule. Diameters 
generally range from 3 to 6 in, but significantly larger 
diameters have been achieved. The CZ method accounts 
for virtually all of the commercial production of single- 
crystal silicon in the United States. The FZ method 
(fig. 8B) starts with a solid polysilicon rod as grown in 
the decomposition reactor. The rod is made to contact a 
seed crystal after a small zone at the seed end is melted 
with an induction coil. The coil is moved slowly along the 
length of the rod, leaving the material behind the molten 
zone solidified as an oriented single-crystal form. FZ 
boules are typically smaller than those prepared by the CZ 
method, ranging from 1 to 3 in. in diameter. Further puri- 
fication can be achieved with both methods, since most 
impurities tend to remain in the molten silicon rather than 
solidify in the single crystal. However, a major advantage 
of the FZ process is that the molten silicon zone is held 
between the two solidified portions of the rod by surface 
tension and is not in contact with any other material from 
which it can pick up impurities. 

CHARACTERIZATION 

A primary concern in the preparation of ultra-high- 
purity silicon is accurate analysis of the material for 
impurity elements. Control of these impurities must be 
maintained throughout the process. Methods of charac- 
terization must be constantly improved and new methods 
developed to keep pace with ever-increasing purity re- 
quirements. Under current technology, characterization 
is generally conducted on chlorosilane, silane, and single- 
crystal silicon. Chlorosilane and silane gases can be mon- 
itored by gas chromatography at certain stages of 
production. To test silicon material, a small sample of 
polysilicon must be FZ refined. Wafers cut from the FZ 
material can be analyzed optically or tested for their elec- 
trical transport properties. The two most common meth- 
ods of optical analysis are (1) Fourier transform photo- 
luminescence spectroscopy for donor/acceptor detection 
and (2) Fourier transform infrared spectroscopy for mea- 
surement of carbon and oxygen. Electrical transport mea- 
surements based upon the Hall effect yield the most sensi- 
tive data on donors and acceptors (e.g., <1 ppt) and are 
used to calibrate the photoluminescence technique. Ac- 
cording to Dr. Patrick M. Hemenger, U.S. Department of 
the Air Force, these methods possess the necessary detec- 
tion limits for present ultra-high-purity specifications. 



cJ^ 




Vent 



L 



Heat shield 



Quartz bell jar 



Polycrystalllne 
silicon U-rod 



Graphite support 



Power electrode 



TCS 

To recovery 

Figure 7.-Polysilicon reactor. 




Growing crystal 

Fused silica liner 






n 



7 /\ Graphite crucible 



# Zone heater 
— Ingot 



Single-crystal seed 



Holder 



B 



Figure 8. -Single-crystal silicon production method. 
A, Czochraiski and 8, Float-zoning. 



SUPPLY AND DEMAND 






Since the supply of ultra-high-purity silicon for IR de- 
tectors depends on the various aspects of a multistep pro- 
cessing chain, the supply of materials involved in each step 
should be examined if a quantitative materials perspective 
is to be gained. However, very few official production or 
production capacity data are available for silanes, poly- 
silicon, or single crystals. The appendix lists the major 
domestic merchant firms involved in the different phases 
of the semiconductor silicon chain, along with some for- 
eign firms that are involved in the more advanced pro- 
cessing stages. 

As discussed previously, raw silica is readily available 
in most parts of the world, and a number of countries 
produce silicon metal on a tonnage basis (table 2). 

The United States produces 120,000 to 160,000 st of 
silicon metal annually. However, demand is normally 
30,000 to 40,000 st higher than production, making the 
United States a net importer of silicon metal. The United 
States does export silicon metal, but the amount is typically 
less than 10% of domestic production (table 3). Both 
production and imports of silicon metal account for rough- 
ly one-third of production and imports of silicon materials 
overall (table 4). 

Production and production capacity for other phases of 
the semiconductor silicon processing chain (i.e., silanes, 
polysilicon, and single-crystal silicon) are adequate in the 
United States. Annual production capacity for high-purity 
polysilicon is about 4,000 st, and the major producers have 



ample capacity to produce the necessary supplies of TCS 
and silane starting materials. However, concerns do exist 
at the latter end of the chain, since only one of the re- 
maining major slice companies (i.e., producers of single- 
crystal boules and wafers) is U.S.-owned, and the sale of 
this company to a foreign buyer is currently under 
consideration. 

On a global basis, annual production capacity for high- 
purity polysilicon is about 10,000 to 12,000 st, outpacing 
demand by 2,000 to 3,000 st. As in the United States, 
foreign producers generally possess adequate supplies of 
TCS and silane starting materials. On the single-crystal 
and wafer end of the spectrum, Japan is clearly the world's 
leader (table 5). 

U.S. military demands for ultra-high-purity polysilicon 
and FZ single-crystal silicon used to make IR detectors 
are approximately 500 and 200 kg, respectively. Currently, 
the demand for both materials is met by foreign sources. 
Foreign suppliers of ultra-high-purity material are listed 
among the world polysilicon and single-crystal producers 
in the appendix. Although ultra-high-purity polysilicon can 
be produced domestically, and FZ equipment does exist, 
neither material is produced on a regular basis. However, 
current capabilities are being improved, and more regular 
supplies of ultra-high-purity polysilicon as well as FZ single 
crystals are being developed under Government-sponsored 
programs. 



Table 2. -World silicon metal production 

(Thousand short tons) 



1987 p 



Country 



1983 



1984 



1985 



1986 



-1988' 



Brazil 

Canada 

China 

France 

Italy 

Norway 

South Africa, Republic of 

Spain 

Sweden 

U.S.S.R 

United States 

Yugoslavia 

Other 

Total 

p Preliminary. 
Estimated. 



23 
28 
24 
72 
15 
85 
30 
19 
22 
70 
122 
29 
43 



582 



30 
28 
24 
78 

15 
100 
38 
66 
22 
70 
141 
31 
43 



32 
28 
30 
77 
15 

112 
39 
68 
22 
66 

121 
36 
45 



686 



691 



41 
29 
45 
77 
13 

110 
39 
68 
22 
72 

124 
35 
42 



717 



44 
33 
70 
77 
13 

110 
37 
77 
22 
72 

147 
35 
40 



777 



87 
33 
75 
77 
13 

110 
37 
77 
22 
72 

164 
55 
36 



858 



10 



Table 3.-U.S. silicon metal statistics 

(Thousand short tons, gross weight) 



1983 



1984 



1985 



1986 



1987 



1988 



Production 
Imports . . 
Exports . . 



123,602 

28,173 

2,767 



144,005 

25,221 

4,420 



122,787 

51,801 

2,120 



125,966 

40,851 

5,378 



150,080 

36,930 

9,247 



164,348 
62,030 
10,304 



Table 4. -Silicon supply-demand relationships and prices, including silicon metal 
and silicon-containing ferroalloys 

(Thousand short tons of Si) 



1983 



1984 



1985 



1986 



1987 



1988 



PRODUCTION 



United States 333 454 400 

Rest of world 6 2,511 2,770 2,717 

Total 6 2,844 3,224 3,117 

COMPONENTS AND DISTRIBUTION OF U.S. SUPPLY 

Components: 

Primary production 333 454 400 

Imports 133 121 154 

Industry stocks-Jan. 1 109 86 90 

Total 575 661 644 

Distribution: 

Industry stocks-Dec. 31 86 90 102 

Exports 9 20 9 

Industrial demand 480 551 533 

U.S. DEMAND PATTERN 

Chemicals 62 99 101 

Construction 62 77 75 

Machinery 101 94 91 

Transportation 154 171 165 

Other 101 VI0 101 

Total 480 551 533 

AVERAGE ANNUAL FREE-MARKET PRICES 

Regular-grade 50% ferrosilicon, 

actual (cents per pound Si) 36.1 40.7 36.6 

Regular-grade 50% ferrosilicon, 

based on constant 1988 dollars 

(cents per pound Si) 42.3 46.0 40.2 

Regular-grade 75% ferrosilicon, 

actual (cents per pound Si) 36.0 41.3 35.0 

Regular-grade 75% ferrosilicon, 

based on constant 1988 dollars 

(cents per pound Si) 42.2 46.7 38.4 

Regular-grade silicon metal, 

actual (cents per pound) 36.0 41.3 35.0 

Regular-grade silicon metal, 

based on constant 1988 dollars 

(cents per pound) 42.2 46.7 38.4 

Estimated. 

NA Not available. 



335 
2,686 



3,021 



628 



527 



34.8 



35.2 



373 
2,668 



3,041 



634 



560 



41.3 



41.7 



465 
NA 



NA 



335 


373 


463 


191 


189 


212 


102 


72 


57 



735 



72 


57 


49 


11 


17 


27 


527 


560 


658 


105 


118 


NA 


69 


73 


NA 


90 


95 


NA 


169 


179 


NA 


95 


95 


NA 



NA 



51.0 



37.2 


42.7 


51.0 


32.9 


40.3 


55.6 


35.2 


41.7 


55.6 


32.9 


40.3 


55.6 



55.6 



Source: Bureau of Mines. 



11 



Table 5.-World semiconductor silicon suppliers 

(Silicon and epitaxial wafer sales in million dollars) 



Company 

Shin-Etsu Handotai (Japan) 

Mitsubishi Metal (Japan) 

Osaka Titanium Co. (Japan) 

Wacher (Germany, Federal Republic of) 
Komatsu Electroonic Metals (Japan) . . . 

Monsanto (United States) 

Other 

Total 

NA Not available. 

Source: Dataquest. 



1985 



1986 



1987 



1988 



$310.0 


$408.0 


$452.1 


128.0 


195.0 


241.3 


160.0 


197.6 


235.5 


205.0 


194.6 


166.5 


116.0 


168.5 


197.3 


137.0 


154.0 


185.0 


210.5 


233.8 


249.3 



1,266.5 



1,551.5 



1,727.0 



NA 
NA 
NA 
NA 
NA 
NA 
NA 



$2,172.0 



RESEARCH AND DEVELOPMENT 



Probably the most current domestic research aimed at 
developing a more stable (i.e., production on a regular 
basis and from a domestic source) supply situation for IR- 
detector-grade silicon in the United States involves two 
Government-sponsored programs. One is the Air Force's 



recently completed "Boron-Free Silicon Detectors 
Program", and the other is a title III contract under the 
Defense Production Act (DPA) calling for the develop- 
ment of a domestic ultra-high-purity silicon source. 



GOVERNMENT PROGRAMS 



In 1984, a project involving Hughes Aircraft Co. and 
Union Carbide Corp. was initiated under Air Force 
Contract F33615-84-C-5025 to produce starting materials, 
including silane and polysilicon, to be used in the manu- 
facture of extrinsic silicon IR detectors. This work, 
known as the Boron-Free Silicon Detectors Program, was 
completed in 1988 and concluded that, through Union 
Carbide's modified silane process, ultra-high-purity silicon 
could be produced on a regular basis. Also as a result of 
the program, much improved characterization techniques 
(i.e., impurity analysis) were developed (10). 

Under title III of the DPA, President Reagan granted 
the Defense Department $3 million in 1986 for 
Government-sponsored development of an ultra- 
high-purity silicon industry. Title III of DPA gives the 



Administration the authority to expand capacity or develop 
technological processes through the use of loans and guar- 
anteed purchase contracts. Hemlock Semiconductor Corp. 
in Hemlock, MI, recently completed a title III contract 
(phase I) under which 150 kg of ultra-high-purity poly- 
silicon was to be produced. Phase II of this title III 
program is to be awarded to Hemlock sometime in 1989, 
pending complete verification that phase I specifications 
were met. The phase II contract calls for 1,000 kg/yr of 
Phase I-grade polysilicon for each of the next 3 yr. At the 
same time, title III contracts will be awarded to convert 
100 kg of the phase I material by the FZ method. The 
other 50 kg produced during phase I was used for ana- 
lytical purposes. The FZ contracts will be limited to small 
companies (i.e., less than 500 employees). 



12 



SUMMARY 



Both the raw materials necessary to produce silicon on 
a tonnage basis and production capacity are abundant in 
the United States and other parts of the world. Ample 
production capacity for the silicon-based chemicals needed 
for semiconductor silicon also exists. However, the indi- 
vidual stages involved in the processing of raw silica into 
the ultra-high-purity silicon needed for the manufacture of 
today's IR detectors can be quite complex, and domestic 
production in the latter stages is uncertain. The purity 
requirements of such materials are very stringent at most 
stages, and the control of impurity elements will become 
even more crucial in the manufacture of more advanced 
devices in the future. Currently, preparation of ultra-high- 
purity silicon for IR detectors can be achieved domestically 



through the polysilicon stage. However, there is no do- 
mestic production on a regular basis. Many questions as 
to the establishment of long-term domestic FZ capabilities 
also exist, since there is currently no merchant FZ produc- 
tion in the United States. Characterization of ultra-high- 
purity silicon is a continuing concern. New and improved 
characterization techniques must be developed to keep 
pace with device requirements. Ongoing Government- 
sponsored programs have closed some of the gaps in the 
domestic ultra-high-purity silicon processing chain. How- 
ever, the United States continues to depend on foreign 
sources for a stable supply of this material for its defense 
needs. 



REFERENCES 



1. Kirk-Othmer Encyclopedia of Chemical Technology. V. 17, 3d 
ed., 1980, p. 560. 

2. Business Week. No. 2970, Oct. 27, 1986, p. 46. 

3. Van Vlack, L. H. Electron Transport in Solids. Ch. in Elements 
of Materials Science and Engineering, ed. by M. Cohen. Addison- 
Wesley, 4th ed., 1980, pp. 149-182. 

4. Smith, P. C. Private communication. Data in brochure available 
from Advanced Technology Div., Westinghouse Electric Corp., 
Baltimore, MD. 

5. Jackson, H. W. Insulators, Semiconductors, and Sources of EMF. 
Ch. in Introduction to Electric Circuits. Prentice-Hall, 4th ed., 1986, 
pp. 38-63. 

6. Robiette, A. G. E. Manufacture of Silicon Alloys. Ch. in Electric 
Smelting Processes. Halsted Press, 1973, pp. 102-125. 



7. McCormick, J. R Polycrystalline Silicon. Paper in 
Semiconductor Silicon 1986, ed. by H. R Huff, B. Kolbesen, and 
T. Abe. Electrochem. Soc., 1986, pp. 46^8. 

8. Taylor, P. A. Silane: Manufacture and Applications. Solid State 
Technol., v. 30, No. 7, 1987, pp. 53-59. 

9. Crossman, L. D., and J. A. Baker. Polysilicon Technology. Paper 
in Semiconductor Silicon 1977, ed. by H. R Huff and E. Sirtl. 
Electrochem. Soc., 1977. 

10. Gittere, W. J. Private communication. Data in brochure 
available from Market Development and Sales, Union Carbide Corp., 
Tonowanda, NY. 

11. Robertson, G. D., M. H. Young, J. P. Baukus, O. J. Marsh, and 
R N. Flagella. High Purity Silicon for Detectors. IRIA IR Materials 
Conf., 1988, Palo Alto, CA. 



13 



APPENDIX.-MAJOR DOMESTIC AND FOREIGN FIRMS INVOLVED 
IN SEMICONDUCTOR SILICON PRODUCTION 



Silicon metal 



Silanes 



Domestic 

Dow Corning 
Elkem Metals 
Globe Metallurgical 
Silicon Metaltech 
Simetco 
SKW 

Dow Corning 

Ethyl 

Union Carbide 



Foreign 

Numerous companies 
worldwide produce 
silicon metal. 
See table 2 for 
production by 
country. 

Komatsu 

Shin-Etsu 

Wacher 



Polysilicon 



Ethyl 

Hemlock Semiconductor 
(63% owned by Dow 
Chemical, 37% by 
Japanese firms) 

Union Carbide 



Dyamit-Nobel 

Komatsu 

Mitsubishi Metal 

NKK 

Rhone Poulenc 

Shin-Etsu 

Tokuyama 

TopsU 

Wacher 



Single crystal 
and wafers. 



Crysteco 

Monsanto 

NBK (owned by Kawasaki 

of Japan) 
Siltec (owned by 

Misubishi Metal of 

Japan) 



Kamatsu 

Mitsubishi Metal 

Monsanto 

Osaka Titanium Co. 

Shin-Etsu 

Topsil 

Wacher 



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